Summary

As naturally occurring large bodies of freshwater are rare in South Africa there are numerous man-made dams and lakes, these systems are highly vulnerable to colonization from non-native invasive plants due to multiple factors. Firstly, there is a lack of native aquatic plant species to occupy the water column and compete for resources. Secondly they are often eutrophic systems, caused by anthropogenic activity such as intensive agriculture and improper human waste disposal, and nutrient loading is a known driver of invasive plants. Also, it has been argued that the intrinsic nature of freshwater systems leads them to be disproportionately affected by non native invasive species when compared with terrestrial systems (Moorhouse and McDonald 2015).

This combination has lead to a long battle against floating invasive plants that dominate many of South Africa’s freshwater resources.. These plants form dense mats on the waters surface, restricting light to other species, damaging hydroelectric equipment, limiting water quality and reducing biodiversity. They can also play host to vectors of disease such as malari and schistosomiasis (Mack and Smith, 2011). The implementation of classical biological control programs, using the natural enemies of the invasive plants, has proven to be a huge success when controlling detrimental invasive plants such as water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes L. (Araceae)), Kariba weed (Salvinia molesta D.S. Mitchell (Salviniaceae)), parrots feather (Myriophyllum aquaticum (Vellozo Conceição) Verdcourt) and red water fern (Azolla filiculoides Lamarck (Azollaceae)) (Hill 2002)

The host-specific biological control agents (BCAs), typically insects and mites, have coevolved with the plants in their natural range and are intentionally introduced to manage invasive plant populations.

The overall aim of this is to induce a regime shift into a functioning system with high native biodiversity and freshwater access. However, we propose that whilst the BCAs do lead to a dramatic reduction in the biomass and health of the floating plant, it can also act as a catalyst inducing a shift into a second degraded stable regime. This second regime is one that is dominated by submerged invasive plants. The establishment of the BCAs on the floating plants can lead to a rapid plant population crash and the nutrients they were locking up are released. At the same time submerged light levels are restored in the water column, enabling a new suite of submerged invasive plants to flourish. The increase in space, light and nutrients promotes the submerged plant growth and as they continue to photosynthesize the levels of dissolved oxygen in the water rise. This improvement in water quality, alongside a limited number of native submerged plants to compete with, helps to establish and maintain this second stable regime (invasive submerged plant dominance). These rooted plants can alter water flow, turbidity and sediment stabilization (Yarrow et al. 2009). They can also degrade water quality and biodiversity, restrict access to freshwater and damage hydro-electrical equipment.

Key Attributes

Spatial scale of RS

Time scale of RS

Reversibility

Evidence

Confidence: Existence of RS

Speculative – Regime shift has been proposed, but little evidence as yet

Confidence: Mechanism underlying RS

Well established – Wide agreement on the underlying mechanism

Alternate regimes

Invasive floating plant dominance

This regime is characterized by dense, mono-specific mats of free-floating invasive plants that cover vast areas of the water’s surface. The water below lacks adequate light for many species to survive and biodiversity is restricted. These mats can completely conceal the water-body beneath and as they are free floating they can be displaced by flash floods and strong winds, often damaging hydroelectric equipment and causing a sudden change in ecosystem structure and function.

Invasive submerged plant dominance

Dense monocultures of rooted submerged invasive plants dominate the system, often not seen until they reach the water’s surface by which point they are usually problematic. Native vegetation is out competed, and whilst initially these plants can push up levels of dissolved oxygen and improve water quality they rapidly grow to a state where only the plants near the waters’ surface are healthy and just below the water is turbid and hypoxic. Water flow is also limited and pumping mechanisms can become clogged and damaged, increasing flood risk. These plants often reproduce via fragmentation and can easily be spread through river systems by wildlife, people, boats and often by the machines used to mechanically clear the plants from invaded areas.

Drivers and causes of the regime shift

Shift from floating invasive to submerged invasive plant dominance

The main causes of the regime shift, we believe, are the biological control of floating invasive plants in freshwater systems. Nutrients that were previously locked within the floating invasive plants are now available for submerged invasive plants to acquire. Another major factor is the intrinsic nature of these systems, they are man-made and historically not a common feature of South African topography meaning there are fewer native species to readily occupy them. As the floating plants are controlled there may not be a good stock of native submerged plants to utilize the resources and instead resilience is very low in this unstable state. Poorly regulated human activities such as waste water treatment and unregulated agricultural endeavors have also led to problematically high nutrient loading in many of these systems.

How the regime shift worked

Shift from floating invasive to submerged invasive plant dominance

The invasive floating plant regime often occurs in man-made, impounded freshwater systems in South Africa. The invasive plants have often been intentionally introduced via the aquarium and ornamental trade, and un-intentionally via ‘hitchhiking’ on other species and aquatic machinery. In many cases external nutrient loading facilitates their growth and minimizes the impact of biological control agents therefore perpetuating the regime. The floating plants quickly form mats that block light to the water column, which can reduce the presence of other aquatic plants that might compete for resources, again maintaining their dominance. Eventually they can alter the whole ecosystem structure and function diminishing the presence of and access to quality freshwater.

Once invasive floating plants are dominant biological control agents may be introduced to control the floating plants, with the hope of inducing a regime shift to a clear water system with healthy biodiversity and good quality water. However, the regime shift we believe is happening in many systems is quite different. As the floating plants decompose dues to the effects of control agents the system receives a sudden influx of freely available nutrients, and at the same time light levels within the water column are restored. As nutrients, light and space become available submerged invasive plants are able to utilize the resources and establish.

The submerged plants’ ability to photosynthesize temporarily increases the levels of dissolved oxygen in the water column thus improving water quality (short-term) and allowing the submerged plants to become dominant. After this period of improvement, the water quality deteriorates as the plant biomass crosses a threshold after which it begins to block out light, reduce biodiversity and alter sediment stability. External nutrient loading further facilitates their growth and helps to sustain the new invasive submerged plant regime.

Impacts on ecosystem services and human well-being

Shift from floating invasive to submerged invasive plant dominance

This regime shift leads to diminished access to quality freshwater. Livelihoods that are dependent on South Africa’s freshwater biodiversity, such as fishing and eco-tourism are also compromised (Charles and Dukes, 2007). As are all activities that depend on access to freshwater for irrigation and livestock. Many farmers have lost livestock to drowning as they perceive large mats of floating plants to be solid underfoot (McConnachie et al, 2003). Hydroelectric pumps are damaged, once again limiting water access, and the costs related to repairing these and to the mechanical/chemical control of the submerged plants can be substantial.

Besides impacts of economic activities and livelihoods, this regime shift also directly impacts human well-being. Invasive aquatic plants play a key role harboring vectors of diseases such as schistosomiasis (bilharzia) and malaria (Mack and Smith, 2011). Continued mismanagement of invasive plants in South Africa, perpetuated by poor understanding of the systems, leads to incorrect spending of state funding potentially affecting a wider cross-section of people and communities than those directly affected. Protection against natural disasters are also affected, as floods defenses are compromised by the plants which alter water flow and can increase water levels by raising sedimentation.

Management options

Potentially, once the dominating invasive submerged plants are controlled (manually, physically or biologically) there could be a shift back to the floating plant dominance if the environmental conditions that facilitated the floating plants have not changed or been managed and if no ‘reserve’ of the floating plant has been left to support a population of it’s bio-control agents.

By reducing levels of eutrophication alongside the control of invasive floating plants we can increase resilience against colonization from submerged plants. We also propose that increasing local levels of native vegetation (via seed banks and plant stocking) in systems before the control of the floating plants is underway could increase resilience as there would be less resources available for invasive plants to utilize.

Key References

Coetzee et al. 2011 Prospects for the biological control of submerged macrophytes in South Africa. African Entomology : Biological control of invasive alien plants in South Africa (1999 - 2010): Special Issue 2.

Coetzee J & Hill MP. 2012. The role of eutrophication in the biological control of water hyacinth, Eichhornia crassipes, in South Africa. BioControl 57, 247-261.

Hill MP. 2002. The impact and control of alien aquatic vegetation in South African aquatic ecosystems. African Journal of Aquatic Science 28, 19-24